INTRODUCTION

Wheat is the principal agronomic crop of the Mexicali Valley. After the grain is harvested, much of the straw is left in the field. The relatively low available energy value of wheat straw limits its nutritive value for feedlot cattle (Lesoing et al., 1980). However, its functional value as a roughage source is well documented (Ware and Zinn, 2005; Salinas-Chavira et al., 2013). Hence, the level of inclusion of wheat straw in high-concentrate finishing diets is largely a functional constraint, i.e. stimulate chewing and saliva output that buffer the acids produced during ruminal fermentation (Balch, 1958). Feedlot receiving diets typically include approximately 35% roughage to allow adaptation of ruminal bacteria to the more readily fermentable carbohydrates (Berry et al., 2004). Whereas, finishing diets contain 4.5% to 13.5% (dry matter [DM] basis) roughage (Vasconcelos and Galyean, 2007). Both roughage concentration and physical form contribute to normal rumen function (Woodford et al., 1986). Pelletizing forages affects its functionality as roughage. It increases particle density and ruminal passage rate, reducing total chewing activity and rumination (Mertens, 1997; Mertens and Ely, 1979). However, the increased density brought about by pelletizing wheat straw would reduce transportation cost over long distances from areas of wheat production to feedlot centers, and enhance conveyance, mixing, and feed delivery of otherwise bulkier finishing diets. The comparative effects of level of inclusion conventional ground vs pelleting wheat straw on characteristics of digestion of finishing diets for feedlot cattle has not been previously evaluated. Steam-flaking improves starch digestibility over that of dry-rolled corn (Zinn et al., 2002). However, in the absence of adequate roughage the enhanced starch digestibility may reduce ruminal pH (Zinn et al., 1995) to the extent that it depresses organic matter (OM) digestion and associated digestive processes, leading to suboptimal average daily gain and gain efficiency (Owens et al., 1986). The objectives of this experiment were to evaluate the influence of wheat straw level and processing method on characteristics of digestion of dry rolled or steam-flaked corn (SFC)-based finishing diets.

MATERIALS AND METHODS

All procedures involving animal care and management were in accordance with and approved by the University of California, Davis, Animal Use and Care Committee.

Eight Holstein steers (216±48 kg) fitted with ruminal and duodenal cannulas were used in a split plot design consisting of two simultaneous 4×4 Latin squares to evaluate the influence of wheat straw level (7% vs 14%, DM basis) and processing method (ground vs pelleting) on characteristics of digestion of dry rolled and SFC-based finishing diets for feedlot cattle. Wheat straw level and processing method were evaluated in a 2×2 factorial arrangement within each 4×4 Latin square design in which diets contained either dry rolled or SFC (whole plots). Composition of experimental diets is shown in Table 1. Diets included 0.35% chromic oxide as an indigestible marker to estimate nutrient flows and digestibility. Chromic oxide was premixed with minor ingredients (urea, limestone and trace mineral salt) before incorporation into complete mixed diets. The eight treatments were: 7% ground wheat straw (GRWS) with dry rolled corn (DRC); 7% pelleted wheat straw (PEWS) with DRC; 14% GRWS with DRC; 14% PEWS with DRC; 7% GRWS with SFC; 7% PEWS with SFC; 14% GRWS with SFC; 14% PEWS with SFC. GRWS was processed in a hammer mill (Bear Cat #1A-S, Westerns Land and Roller Co., Hastings, NE, USA), equipped with a 3.81 cm screen. Pelleting wheat straw (PEWS, 2×0.5 cm pellets) was produced using a California Pellet Mill. SFC was prepared as follows. A chest situated directly above the rollers (46×61 cm, corrugated) was filled with 441 kg of yellow corn and then brought to a constant temperature of 102°C at atmospheric pressure using steam. Grain was steamed for 20 min before starting the rollers. Approximately 454 kg of the initial steam-processed grain that exited the rollers during the warm-up of the rollers was set aside and not fed to the cattle in this study. The roller tension was adjusted to provide a flake density of 0.31 kg/L. Average retention time in the steam chamber was approximately 30 min. The SFC was allowed to air-dry before use in diet preparation. Dry rolled corn was prepared by rolling corn in the absence of steam with roller tension set to provide a density of 0.58 kg/L.

Steers were housed in individual pens (3.9 m2) with concrete floor covered by neoprene carpet, automatic waterers and individual feed bunks. Diets were fed at 0800 and 2000 h daily. Dry matter intake was restricted to 3.94 kg/d. The experiment consisted of four 14-d periods, consisting of 10 d for diet adjustment plus 4 d for sample collection. During collection, duodenal and fecal samples were taken from each steer twice daily over a period of 4 successive days as follows: d 1, 0750 and 1350 h; d 2, 0900 and 1500 h; d 3, 1050 and 1650 h; and d 4, 1200 and 1800 h. Individual samples consisted of approximately 700 mL of duodenal chyme and 200 g (wet basis) of fecal material. Samples for each steer within each collection period were composited for analysis. During the final day of each collection period, ruminal samples were obtained from each steer via the ruminal cannula at 4 h after feeding. Ruminal fluid pH was determined on freshly collected samples. Samples were then strained through four layers of cheesecloth. Two milliliters of freshly prepared 25% (wt/vol) meta-phosphoric acid was added to 8 mL of strained ruminal fluid. Samples were then centrifuged (17,000×g for 10 min), and supernatant fluid was stored at −20°C for volatile fatty acids (VFA) analysis. Upon completion of the experiment, ruminal fluid was obtained via the ruminal cannula from all steers and composited for isolation of ruminal bacteria via differential centrifugation (Bergen et al., 1968).

Organic matter fermented in the rumen was considered equal to OM intake minus the difference between the amount of total OM reaching the duodenum and MOM reaching the duodenum. Feed N escape to the small intestine was considered equal to total N leaving the abomasum minus ammonia-N, MN and endogenous N (0.195×BW0.75; Ørskov et al., 1986). Methane production (mol/mol glucose equivalent fermented) was estimated based on the theoretical fermentation balance for observed molar distribution of VFA (Wolin, 1960).

RESULTS AND DISCUSSION

Treatment effects on characteristics of digestion and ruminal pH and VFA molar proportions are shown in Table 2 and 3. There were no treatment interactions (p = 0.10 to 0.99) on characteristics of ruminal and total tract digestion.

Wheat straw level (7% vs 14%)

As expected, increasing wheat straw level from 7% to 14% decreased (p<0.05) ruminal and total tract digestion of OM. Wheat straw partially replaced corn grain in the diet. The decrease in OM digestion is expected due to the substitution of corn grain with less digestible wheat straw. This decrease of OM digestion with increased wheat straw inclusion rate is consistent with previous studies (Calderon-Cortes and Zinn, 1996; Salinas-Chavira et al., 2013).

Grain processing (dry rolled vs steam-flaked)

Consistent with previous studies (Zinn et al., 1998), ruminal digestion of OM and starch, and post-ruminal and total tract digestion of OM, starch and N were greater (p<0.01) for steam-flaked than for DRC-based diets. As an aside, observed total tract starch digestion for dry rolled and SFC are also in close agreement (99.5%) with estimated starch digestion (92.9% and 98.1%, respectively) based on fecal starch (starch digestion, % = 100.5–0.649 fecal starch; Zinn et al., 2002), where fecal starch for dry rolled and SFC-diets averaged 3.6% and 11.7%, respectively.

Ruminal NDF digestion was greater (p = 0.02) for dry rolled than for SFC. Decreased ruminal NDF digestion is also a consistent effect of steam-flake vs DRC-based diets (Corona et al., 2006). However, in the present study compensatory post-ruminal digestion sufficed to the extent that total tract digestion of NDF was unaffected (p = 0.94) by corn processing.

There were no interactions (p>0.10) between grain processing and wheat straw treatments on ruminal pH, VFA molar proportions, and estimated methane production. Ruminal pH and total VFA concentration were not affected (p≥0.16) by corn processing method. The lack reduction in ruminal pH with steam-flaked vs DRC-based diets is not generally expected (Zinn et al., 1995; Corona et al., 2006). Although in some instances, the effects of steam-flaking corn on ruminal pH were not appreciable (Zinn et al., 1998). The magnitude of differences may be associated with flake density and/or fineness of grind of dry-process corn.

CONCLUSIONS

Results of this study indicate that at 7% or 14% straw inclusion rate, changes in physical characteristics of wheat straw brought about by pelleting may have a small negative impact on OM digestion of both steam-flaked and dry-rolled corn-based finishing diets. This effect is due to decreased post-ruminal starch digestion. Replacement of ground straw with pelleted straw also may result in decreased ruminal pH, although the effect was more pronounced a 14% than at 7% straw inclusion rate.

Notes

CONFLICT OF INTEREST

We certify that there is no conflict of interest with any financial organization regarding the material discussed in the manuscript.

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